The invention relates to an inductive proximity switch having a coil situated to the rear of an xe2x80x9cactivexe2x80x9d housing wall and belonging to a receiver circuit, its signal changing as a metallic object approaches, the coil being operatively connected to a switching amplifier which changes it switching state when the object reaches a predetermined distance.
A proximity switch of this type is known from DE 40 31 252 C1. The proximity switch there described operates in accordance with the principle in which a coil which is part of an LC oscillator generates an alternating magnetic field, and an additional coil combination which, divided up at various locations in relation to the object to be detected, evaluates the field strength. In that document, the coil combination comprises two coils which are connected one behind the other and wound in opposite senses, in which a differential alternating voltage is induced. This differential alternating voltage is fed back to the input of the oscillator amplifier in such a way that the oscillator changes its oscillation state abruptly as the differential alternating voltage passes through zero. This change is used, with an evaluation circuit, to obtain a switching signal.
Also known in the prior art are inductive proximity switches which are equipped with a coil which is located in a ferrite or carbonyl core. This coil is part of an LC oscillator which generates an alternating magnetic field. The circulating currents induced in the approaching metallic object take energy from the oscillator, which leads to a change in the oscillation state of the oscillator. This change is used to change the switching state of the proximity switch.
Such proximity switches and, in particular, those having air-cored coils or carbonyl cores are used in car-body assembly plants, in which the car bodies are welded together. The welding dust produced during welding, or the welding beads which accumulate there, can be deposited on the xe2x80x9cactivexe2x80x9d housing wall of the proximity switch. Because of the ferromagnetic property of these welding beads or of this welding dust, there is an influence on the oscillation state of the oscillator. Proximity switches having a carbonyl core as part of an LC oscillator are influenced by welding dust on the active surface in such a way that they increase their switching distance and, above a specific level of coverage of the active surface, change their output state erroneously.
Inductive proximity switches which operate with two air-cored coils, that is to say those which evaluate a differential alternating voltage, are likewise negatively influenced as a result of the xe2x80x9cactivexe2x80x9d surface being covered by permeable material, specifically in such a way that they reduce their switching distance. The consequence of this may be that metallic objects approaching the proximity switch are not detected, and therefore the switching state of the proximity switch is erroneous.
The invention is therefore based on the object, in an inductive proximity switch, of reducing the influence on the switching distance of any covering of the xe2x80x9cactivexe2x80x9d surface with welding dust.
The aim is first and foremost that a supplementary sensor is associated with the xe2x80x9cactivexe2x80x9d housing wall. This supplementary sensor advantageously has its maximum sensitivity in the region of the xe2x80x9cactivexe2x80x9d housing wall, so that it is capable of detecting the metallic or ferrite particles substantially independently of the approach of the metallic object. The sensor signal from this supplementary sensor is used at the same time to compensate for the change in the receiver signals caused by the covering in such a way that the proximity switch switches at a constant switching distance, in spite of the covering. It is advantageous if a supplementary coil is situated behind the xe2x80x9cactivexe2x80x9d housing wall. The supplementary coil can be disposed close to and adjacent to the xe2x80x9cactivexe2x80x9d housing wall. The supplementary coil advantageously has a smaller coil diameter than the receiver coil. It is used to compensate electronically for changes in the receiver signals produced by metallic deposits, especially ferritic deposits, on the xe2x80x9cactivexe2x80x9d housing wall. The receiver coil can be an air-cored coil which acts together with a further receiver coil, the difference between the alternating voltages induced in the two coils being used to derive the switching signal. The supplementary coil is then preferably connected in series with but in the opposite winding sense to the first air-cored coil, and can also be an air-cored coil. The number of turns, the diameter and the physical position of the supplementary coil are preferably chosen such that, irrespective of the degree of coverage of the xe2x80x9cactivexe2x80x9d wall surface with metal dust or the like, the differential voltage becomes zero or the oscillation state of the oscillator remains uninfluenced if the switching distance remains constant. In a preferred refinement of the invention, the supplementary coil is situated on a circuit board. In this case, it can be formed by a conductor track etched free in a spiral. The coil of the receiver circuit is preferably also situated on the circuit board. While the coil associated with the receiver circuit is situated radially on the outside, the supplementary coil is situated radially on the inside. In a variant of the invention, the receiver coil can also be situated on a carbonyl core and provided as an oscillator coil of an oscillator. The supplementary coil is then preferably the part of a compensator circuit. By means of this compensator circuit, the degree of coverage of the xe2x80x9cactivexe2x80x9d wall can be determined. This compensation circuit can in turn be part of an evaluation circuit. On the basis of this degree of coverage, determined in this way, the compensator circuit can supply an output signal which is fed as a control signal to a switching amplifier. The switching amplifier switches the output in accordance with the amplitude or the frequency of the high frequency generated by the oscillator. The level of the threshold is influenced by the control signal in such a way that the shift caused by the covering is thereby compensated for.